Manufacture of ferrochromium alloy



Patented J une 1, 1926.

v UNITED: STATES ENGLAND, ASSIGNOB,

AMERICA, A CORPORATION noLMxsFmLn.

301mm WILD, or v conrona'rron or BUSTLESS IRON Manor-scrum: or rannocnnomnm ALLOY.

no nmwiiig. Application filed February The present invention is for improvements in and relating to the manufacture of alloys of chromium and iron, such, for example, as ferro-chrome and metal of the type common 6 ly kpown as stainless or unstainable iron and stee It is already known to'submit moltencrude ferro-ehrome to an oxidizing blow with the object of obtaining an alloy of reduced con- 1 tent in impurities, such as carbon and silicon, which are removable by oxidation. In this operation, oxidation of the chromium takes place to a greater or lesser degree, an the loss of the metal in this way may considerable. The present invention aims at providing an improved processof this kind whereby impurities of the aforesaid type ma be eliminated, or brought down to the deslred lower content, while loss of chromium through oxid tion by the blow is reduced. a

According to this invention, the manufacture of alloys of iron and chromium such, for instance, iron or stainlessrsteel, by a process of the type wherein a crude'basis metal comprising iron and chromium and containing impurities, such as carbon and silicon, removable by oxidation is subjected in the molten state 3 to an oxidizing blow in order to reduce the content of saidimpurities, is characterized by the employment in the iron chromium mixture or alloy submitted to the blow of an additional oxidizableelement, that is to r say, an element additional to. the impurity or impurities to be removed, bon and silicon) and 0 her than silicon, which element is more oxidizable than chromium at the temperature 0 the blowing operation, for the purpose 0 reducing the aforesaid loss of chromium during'oxidatio'n of the impurities. I The preferred additional oxidizable ele- 1 ment isametal which whi (such as carle more oxidizable than chromium is lsuflici entlyl'ess oxidizablef than tlie impurities to be burntzaway bef fbe remo'vedas notto' ore. .of. purification of the l alloy. has. been-cf;

:iec e y Q example, maugan, se. may. be

9 addedto the iron-chromiur'ri mixture orralloy;

- I to be bl0wn,*-'and such additionmayflbe con made inlthefiorm of ferric:inan-v or other suitable: manganesebearing alloy, insuflicient quant ty tolreduce to the as ferro-chrome or stainless-.

the desired degree 5, 192a. Serial No. 61am.

minimum possible the oxidation of the chromium during the subsequent blowing operation. w

Alternatively, the element protective to the chromium may be introduced during the preparation of the basis metal to be blown by reducing a reducible compound of chromium in presence of iron (for instance, by reducing a reducible pound such as chromite) and of said pr0 tective element or of a reducible compound thereof. For instance, the reduction may be carried out in the presence ganese, orof a reducible oxidized manganese compound. 5

Accordmg to the preferred mode of operation of this invention, the im roved ess for the manufacture of a loys of iron and chromium, such as ferro-chrome or stainless-iron or stainless-steel consists in first producing a crude basis metal of a nature speciallyfa-vourable to purification with- 'a blow by reducing a suitable reducible chromium compound in presence of iron (for instance, by reducing a reducible ironohromium com ound such as chromite) with substantially t e chemical equivalent of a silicon-reducer (free fromcarbon or of such content therein'as shall not be detrimental to the final alloy desired, for instance, lowcarbon ferro-silicon) and then submitting the low-carbon-, high-silicon-basis metal so produced, after addition thereto of the requisite 'quantity'of a protective element for the chromium for example, ferro-man nese, orvthe equivalent of another suita 1e manganese alloy, to desilicizing with an oxidizing blow. other conditions being like, silicon? is more conveniently eliminated than carbon,

blast and as silicon-reducers, such, for instance, as ferro-silicon, of low carbon content may be obtained, without difliculty, this preferred method of opera .tion prpvidesfa= -means. taming: a low-'carbonpurified 'product in a simple) and practically;directfmanner; The

by an oxidizing type of basis" metal. roduced' by silicon :IO: Lducti'on cannotbe obtained dire tl' from the, at M ma b a .fi rn e r h t pa ara a t a which jt 'fr l i i x also sufiicientftdsenablethe oxidationgoi the: impurities in'the crude. alloy. by-an'air-bl'ast prddeed without necessity BY MESNE' ASSIGNMENTS, TO

of geconomieallyz ob takes place ream -Q iron-chromium comof ferro-manion ! bath of molten dition of a special reagent'to raise the tem' perature for the blowmg operation, but in the absence of a protective element of the type aforesaid the usual disadvantage is ex perienced of undue loss of chromium by oxidation.

Alternatively, the reduction with the silicon-reducer may be effected as aforesaid in presence of the element which will be protective to the chromium; or silico-manganese may be employed as reducer, in which case the reducing agent may itself provide the protective element required.

.Stainless chrome-iron or chrome-steel may be directly prepared by effecting the reduction of the chromium-compound (preferably with a siliconreducer as aforesaid) within a iron or steel substantially free from carbon, or of such content therein as shall of being proportioned so as to provide a basis metal for the blow of the metallic composiprovide for the desired proportion tion to yield as the purified product a stainless iron-chromium alloy metal. Or a ferrochrome may be initially produced and then fused with the appropriate quantity of iron or steel, and the metal submitted 'to the blow. Or the ferro-chrome may be blown under the prescribed conditions prior tofusion with the iron or steel. In each case, the protective element may be introduced at any convenient stage prior to the blow, for

example, during the preparation of the basis metal in the manner described.

The proportlon of manganese to be util ized for the purpose of protecting the chro- "blow depends upon circumstances, such as as the purified product a stainless chromeiron or chrome-steel, i. e. a metal of a chromium content substantially between 9 and 16 per cent, it may be said that the minimum proportion of manganese to protect the "chromium is the quantity represented by approximatelytwo-thirds of the proportion of the total oxidizable impurities, such as carbon and silicon and not substantially less than 2.5 per cent where said impurities are Iron-chromium alloys of a higher'chromium content than that of stainless metal, for example, ferro-chrome do not behave analogously to the former metal in this respect, and in spite of the -""l1igher content in chromium the proportion of manganese relatively thereto may be comparatively small. With an increase in the carbon content of the fused metal, it may be found advisable to increase the proportion of manganese in the case both of metal of. the composition of stainless-iron or staincarbon in the final alloy metal, said bath necessary mlum in the basis metal submitted to the less-steel and of iron-chromium alloys of a higher chromium content such as ferrochrome.

The reduction mixture may be made up in the form of briquettes, which are then heated to the temperature necessary to start the reaction. The heat evolved by combination of the oxides and the silicon-reducer enables the operation then to proceed with the minimum application of external heat. Suitable fluxes, such as lime and fluors ar, may be employed to facilitate the reaction. The reduction with ferrosilicon is preferably effected in presence of a reagent, such as limestone or lime, which is capable of combining with the oxidized silicon and forming a slag therewith.

A ferro-silicon of high silicon content, for example, 80 to 90 per cent, of silicon, is the preferred form of this reagent, as therebg a more energetic reaction is brought a out, and a product of-highercontent in the metal or'metals other than the iron is obtained.

The reduction may be carried out in any suitable furnace in which the necessary high temperature is attainable, and -it has been found convenient to use an electric furnace, for instance, of the Snyder or Heroult type. For small-bulk charges, an ordinary crucible furnace may be employed. External heating may be applied in order to maintain the high temperature throughout the reduction. Like considerations as to the maintenance of requisite high temperature conditions apply to the blowing operation.

I have found that a .inethod of carr is to fuse the reducer with a part of the char e of the slag-forming reagent, such as limestone, either alone orin presence of a bath of iron or steel, and then to add the reducible charge to the melt, preferably in successive quantities, the metallurgical reactionlbeing assisted by agitation of the bath by rabbling, and by the addition of the remainder of the slag-forming reagent as re uired.

When the ebullition pro need in the bath by the reducing action has subsided, and providing the element protective to the chromium has not'already been introduced duringthe reduction, the addition of this element is now made. For instance, ferromanganese may be added tothe bath, and when the whole mass has been brought to a state of fusion, the iron-chromium alloy, containing silicon, is submitted to the oxidizing blow.

A convenient and economical way of deing the invention into effect livering the air blast into the molten metal carbon and manganese,

generally convenient erro-silicon or other silicon- I is by means of a wrought-iron pipe, cov-' ered with fireclay sleeves, and jointed with fireclay. This pipe may lead into the furnacc directly on to the hearth thereof.-

draught. For the .ient stage of the process, to

' naeeaseo The blow may be a blast of cold or pre heated air under a forced or suction purpose of curtailing the period of the blow, or, at any oonven assist in the. metal, the

oxidation of the impurities in the blast may advantageously be caused to compound of chromium,

i high-chromium ferro-chrome,

or other reducible oxidized or a basesuch as into the Example I.

A: charge of 54 lbs. of ferro-silicon, assaying 83 .per cent silicon and 0.28 per cent,

carbomwas fused in a Snyder electric fur-.

nace and .there were added, in successive charges, 310 lbs. .of chrome-iron ore, assaylbs. of chromium 'SBSQUIOXIQG.

Assay of the metal on completion of the reduction gave:.

Per cent.

I Chromium 67.00 Silicon .04 Carbon s 0.13

v To thisinitialmetal the-re were added 3 lbs. of ferro-manganese, containing 6 per cent of carbon and 76 per cent of manganese whereby 0.15 per cent of carbon and 1.9 per cent of manganese were introduced into the bath.

Aftercomplete fusion, this bath "assayed approximately;-

4 Per cent. Chromium 67.00 Silicon 6.04:

- Carbon 0. 28 Manganese 1.90

An oxidizing blow was then passed. through the molten mass for a erio'd of two minutes, whereby there was 0 tained a final alloy assaying:

Percent Chromium 68.00 Silicon 1.60 Carbon 0.112 Manganese f 0.30

. Elm-ample I.

To abath prepared by fusing 4 cwt. of

lbs. of chrome-iron ore, were added 63 lbs; of silico-manganese' c0n-.

steel scrap with tion, an

'tions, such, for instance,

the ch rome-iron spar was made as required from time to time.

The reduced metal gave on analysis:

Per cent. 12.81 '8. 63 0.20 5. 83

The molten mass, was then submitted to a, blast of cold air, and prior to tapping, there were added to the blown metal 4 lbs. of ferro-manganese together with 4 ozs. of aluminium. The metal was run out into Chromium Silicon Carbon Manganese.-

ing "52.8 per cent chromium sesquioxide togetlier with 30 lbs. of lime. and eslbs. or the mums and assayedr Per cent fiuorspar. 'In this bath the silicon content, Chromium, 13 derived from the ferro-silicon, was equiva-. Silicon "T'f'" lent to 162 lbs. sesquioxide of chromium, the Carbon '5' chrome-iron ore employed representing 163 'g'f" The yield of stainless metal so obtainedwas3cwts.1qr.8lbs.

The above examples are given for illustrative purposes and not by way of limitad t will be appreciated by those skilled in the art that the-operative condias the'choice and per cent of manganese and 25 followed by the addition of a fur A proportions of the special reagents, will necessarily be adjusted according to the circumstances" of any. particular case, and the type of furnace available. These conditions may, however, be readily ascertained bya 'sim 1e. preliminary tria ,T e use of a metal such as manganese has the effect of maintainingthe thermal equilibrium necessary for satisfactory oxidation of the im urities in the, initial alloy by the blow, an mium. .The'presence of a certain quantity of manganese in ferro-chrome is also of ad vantage in that when the alloy is added to a bath of steel or iron the manganese assists deoxidation of the metal, and thereby tends to prevent any further loss of chromium 'throdigh conversion into oxide. 'We have foun that by operating according to this invention, the SillOOIl and carbon impurities can-be eliminated to within the limits of analyses of commercial low-carbon and bo'n-iree ferro-chromium-alloys without substantial loss of chromi'unr due to oxidation.

It will be understood that'the' additional oxidizable element added to the farm-chrotends to provide a'fluid sla as. well :as to impede the oxidation of thee rothe presence of the bath and chromium, the

3 and chromium,

' 50 substantially less than 2% mium alloy to impede the oxidation of the chromium during the blowing operation must not be onerwhose presence in the final purified metal' will be detrimental to the 5 latter in any particular application for which it is intended. Thus, element, care must be observed inthe'case of an alloy of the composition of unstainable.

proportion 10 of tungsten in the finished alloyas otherproperties of the latter is protected from oxidationduring the blow- .ing operation.

2.111 the manufacture of an alloy of iron and chromium, the method of removing impurities from a ferrous bath containing chromium consisting in submitting the bath to an presence of manganese,

oxidizing blow in the the manganese present constituting not substantially less than 2% of the bath.-

3. In the manufacture of an alloy of iron the method of removing silicon existing as an impurity in a ferrous bath containing chromium, consistin in submitting the bath to an oxidizing hlow in the presence of manganese to an amount not of the bath..

4:. The process of manufacture of an unstainable iron-chromium alloy metal which comprises subjecting a molten crude alloy of these metals containing oxidizable im- 4 puritles and of a chromium content between substantially 9-16 per cent to an oxidizing iftungsten be said,

. purities-and It will be understood that the reduction:

eeeeeo blow in presence of manganese in quantity not initially below substantially 2.5 per cent.'.

5. The process of manufacture of an unstainableiron chromium alloy'metal which comprises subjecting a molten crude alloy of these metals containing oxidizable imof a chromium content between substantially 9 to 16 per cent to an oxidizing blow in presence of an initial minimum quantity of manganese represented by approximately two-thirds of the proportion of the total oxidizable impurities present in said crude metal tially 2.5 per cent of manganese.

6. The process'of'manufacture' of an unstainable iron-chromium alloy metal which comprises preparing a low-carbon basis alloy of a chromium content between substantially 9 to 16 per cent by reducing a reducible chromium compound with a low-carbon silicon reducer within a ferrous bathfand sub jecting the low-carbon basis-alloy so produced to desilicizing with an oxidizing blow in presence of an initial minimum quantity of manganese represented by approximately two-thirds of the total oxidizable impurities present in said basis metal and not less than substantially 2.5 per cent of manganese.

7. The process of manufacture of an unstainable iron-chromium alloy metal which comprises preparing a low' carbon basis alloy by reducing chromite with a low-carbon silicon reducer within a.fenrous bath proportioned to give a basis 'al oy containing between substantially- 9-16 per cent of chr0- miuin, and then subjecting the low-carbon basis metal so produced to des ilicizing. with an oxidizing blow inpresence of an initial minimum quantity of manganese represented by approximately two-thirds of the total oxidizable impurities present .in said basis and not less than substanmetal and not less than substantially 2.? per cent of manganese.

In testimony whereofv I have signed my name to this specification.

RONALDLWILD. 

